Miller died Wednesday in his home city of Palm Harbor, the Pinellas County Sheriff's Office said.
Police did not say specifically where Miller died.
Miller spent seven years in the majors, playing for the Blue Jays, Marlins and Giants before wrapping up his career with the Los Angeles Dodgers in 2010.
Miller compiled a 24-14 record and 4.82 ERA in his career, but he may be best known for his role in the 2004 rule requiring pitchers with tattoos on the forearms to wear long sleeves.
The so-called Justin Miller Rule was drafted after opposing hitters complained his colorful tats were a distraction, the Tampa Bay (Fla.) Times said.
After retiring, Miller opened a baseball training center for young ballplayers in Clearwater. His wife, Jessica, told the Times coaching had become her husband's passion.